1. The Field of the Invention
The present invention relates to the fabrication of integrated circuits. More particularly, the present invention relates to an anti-reflective enhancement for integrated circuit fabrication. In particular, the present invention relates to an anti-reflective enhancement for reducing critical dimension loss during mask patterning. More particularly, the present invention relates to formation of a metal silicon nitride antireflective coating layer that resist xe2x80x9cfoot poisoningxe2x80x9d of a masking layer and its detrimental effects.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. In the context of this document, the term xe2x80x9csemiconductive substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term substrate refers to any supporting structure including but not limited to the semiconductive substrates described above.
In the microelectronics industry, the process of miniaturization entails shrinking the size of individual semiconductor devices and crowding more semiconductor devices within a given unit area. With miniaturization, problems arise such as proper electrical isolation between components. Attempts to isolate components from each other in the prior art arm constrained by photolithographic limits of about 0.25 microns. One way to form structures that electrically isolate conductive materials on a semiconductor substrate from each other is to use photolithography in patterning dielectrics layers upon the semiconductor substrate.
To form a metallization wiring layer on a semiconductor substrate by photolithography, a photoresist mask is used to pattern the metallization wiring layer. The mask has directed therethrough a beam of light, such as ultraviolet (UV) light and deep UV (DUV) light (xcx9c250 nm), to transfer a pattern through an imaging lens from a photolithographic template to a photoresist coating which has been applied to the structural layer being patterned. The pattern of the photolithographic template includes opaque and transparent regions with selected shapes that match, respectively, openings and intact portions intended to be formed into the photoresist coating. The photolithographic template is conventionally designed by computer assisted drafting and is of a much larger size than the semiconductor substrate on which the photoresist coating is located. Light is directed through the photolithographic template and is focused on the photoresist coating in a manner that reduces the pattern of the photolithographic template to the size of the photolithographic coating and that develops the portions of the photoresist coating that are unasked and are intended to remain. The undeveloped portions are thereafter remove. Other photolithographic techniques for formation of device features are also possible.
As dimensions shrink below about 0.25 microns, the prior art technique of forming metallization wiring layers becomes more difficult to achieve. Light that is reflected during exposure of a photoresist tends to blur the boundary between two metallization lines and the space therebetween. This blur can cause wider metallization lines than designed, which excessive width will either bridge and short out the circuit or will cause unwanted xe2x80x9ccross talkxe2x80x9d such that the device is rendered defective.
In general, the blurred edge of a critically dimensioned photoresist layer caused by reflected light in photolithographic techniques also result in problems in contact corridors, vias, wiring trenches, and isolation trenches, where the dimensions are patterned below about 0.25 microns. For example, a contact corridor that is too wide will cause notching into a gate stack during a contact corridor etch. Notching causes encroachment into conductive areas of an adjacent gate stack and filling the contact corridor with metallization material can cause a short to occur between the contact and the conductive elements of the adjacent gate stack. A wiring trench that is too wide will cause xe2x80x9ccross talkxe2x80x9d with the wiring in a neighboring trench so as so to compromise and accuracy of the integrated circuit associated therewith.
The resolution with which a pattern can be transferred to the photoresist coating from the photolithographic template is currently limited in commercial applications to widths of about 0.25 microns. In turn, the dimensions of the openings and intact regions of the photoresist mask, and consequently the dimensions of the shaped structures that are formed with the use of the photoresist mask, are correspondingly limited. Photolithographic resolution limits are thus a barrier to further miniaturization of integrated circuits. Accordingly, a need exists for an improved method of forming semiconductor device features that have a size that is reduced from what can be formed with conventional photolithography.
During photolithography, reflected light that occurs during exposure of a mask tends to blur the desired image because the reflected light escapes beyond exposed regions on the photoresist. The blurring problem is caused by reflected light affecting areas of the photoresist that are outside the design pattern.
FIG. 1 illustrates the problem of blurring caused by reflected light that occurs during exposure of a photoresist. A semiconductor structure 10 may be, for example, a semiconductor substrate 12 that was designed to have a width D, but due to blurring caused by reflectivity of patterning light from structures beneath the photoresist, semiconductor substrate 12 has an actual width A. The variance between design width D and actual width A is illustrated as the distance 2(B/2) or B. By way of example, semiconductor substrate 12 was designed to have a width D of 10 in arbitrary units, but due to blurring caused from reflectivity, the actual width A is nine in arbitrary units. It can be seen that a ten percent variance between design and actual width has occurred.
As miniaturization technology continues, a blurring variance of B as illustrated in FIG. 1 will increase relative to an ever-decreasing design width D. Thus, as also illustrated in FIG. 1, a miniaturized semiconductor substrate 12xe2x80x2 that may have a design width Dxe2x80x2 of to two and one-half in arbitrary units but with the variance of B, will have the effect of causing a 40 percent error. A variance of B may leave insufficient space upon miniaturized semiconductor substrate 12xe2x80x2 to form desired contacts or structures. It can be seen from the demonstration illustrated in FIG. 1 that the need to eliminate or substantially reduce blurring must keep pace with miniaturization.
Another hindrance to photolithographic limitations are conventional antireflective coating (ARC) schemes. Prior art methods for avoiding reflected light and its photoresist blurring problems include using layers such as titanium nitride or organic materials that reduce the reflected light in order to better control resolution of the photoresist. As the ever-increasing pressure to miniaturize bears upon the microelectronics industry, the conventional antireflective enhancements such as a titanium nitride layer, organic layers, or other layers known in the art are proving inadequate at resolutions below about 0.25 microns.
One problem at a dimension below about 0.25 microns is that of fouling caused by titanium nitride or organic materials. Fouling is defined as a tendency for a selected antireflective layer to resist staying within preferred boundaries. Resistance to staying within preferred boundaries tends to cause photolithographic techniques to be compromised.
When the ARC is a polymer film, it is applied directly to the semiconductor structure to a thickness of about 0.5 microns and photoresist is deposited on top of the ARC. The ARC then has the function of absorbing most of the radiation used during exposure of the photoresist that penetrates the photoresist material. Both standing wave effects and destructive scattering of light due to topographical features are suppressed with use of the ARC. A disadvantage of a polymer film ARC is that the process is increased in complexity and dimensional control may be lost. A polymer film ARC requires application by spin coating of the ARC material and pre-baking of same before applying the photoresist material. A problem of removing the ARC exists following an etch. For example, during anisotropic etching, portions of a photoresist are mobilized and form a liner within a recess that is being etched that further assists in achieving the anisotropic etch. Due to the anisotropic etch, however, the photoresist that was mobilized may have mingled with other elements that cause it to resist removal by conventional stripping techniques. This resistance to stripping requires stripping solutions that have a chemical intensity that may detrimentally effect the structure that was achieved during the anisotropic etch. As such, use of a substance that is intended to aid anti-reflectivity can result in the benefit thereof being mitigated by the requirement of a more chemically intensive stripping solution treatment.
Various attempts have been made to form antireflective coatings in order to further enhance miniaturization. One type of antireflective coating that has been developed includes metal nitrides, such as titanium nitride, and metal silicon nitrides. The prior art use of metal silicon nitrides and metal nitrides was developed for resolution limits at or above about 1.0 microns. At that resolution limit, there was little or no concern about the phenomenon called xe2x80x9cfoot poisoningxe2x80x9d of the photoresist. Foot poisoning is the phenomenon of diffusion of a constituent of the antireflective layer out of the antireflective layer and into the photoresist material. Foot poisoning has the problem of changing the physical qualities of the photoresist material during processing so as to cause the photoresist material immediately adjacent to the antireflective layer to spread or otherwise change. FIGS. 2-4 illustrate the phenomenon of foot poisoning as it develops during photoresist processing. In FIG. 2 it can be seen that semiconductor structure 10 includes semiconductor substrate 12. Upon semiconductor substrate 12 there may be an insulation layer 14 such as borophosphosilicate glass (BPSG), or a silicate formed from tetmethylortosilicate (MOS) decomposition, or the like. Upon insulation layer 14 there is disposed a metallization layer 16 that is to be patterned into a system of superficial metallization lines. A prior art metal silicide or metal silicon nitride antireflective layer 18 is disposed upon metallization layer 16 and a masking layer 20 is disposed upon antireflective layer 18.
During processing of masking layer, as seen in FIG. 3, a critical dimension DC is formed by exposing masking layer 20 to form a patterned mask 22. During curing of patterned mask 22, nitrogen diffuses from antireflective layer 18 into patterned mask 22 and causes patterned mask 22 to expand at the interface between patterned mask 22 and antireflective layer 18. As seen in FIG. 4, patterned mask 22 has formed a foot-poisoned mask 24 in which the critical dimension DC has been lost and an actual dimension, DA has resulted. When critical dimensions are in the range of about 0.5 to 1 microns, foot poisoning may not be a major concern. However, the trend of miniaturization has progressed to the point at which a resulting DA in lieu of DC is an undesirable variance. The need to reduce or eliminate foot poisoning can be appreciated as analogous to the need to reduce or eliminate blurring as illustrated in FIG. 1. In other words, foot poisoning effects must be reduced in a manner that keeps pace with the process of miniaturization.
Another method of attempting to avoid reflected light is to use a metallic mask. Metallic materials, however, can cause contamination of the semiconductor structure beneath due to the high mobility of metal ions in wet chemical environments or in dry-etch vapors. Additionally, although a metallic mask may remain as part of a finished semiconductor structure, a metallic mask may not be able to properly withstand high processing temperatures sometimes required to achieve a preferred semiconductor structure.
What is needed is an antireflective coating scheme that does not substantially add to fabrication cost and does not substantially reduce fabrication yield. What is also needed is an antireflective coating scheme that imparts an antireflective quality to photolithographic techniques not previously achieved in the prior art. What is also needed is an antireflective coating scheme that does not cause fouling of the semiconductor structure. Additionally, what is needed is an antireflective coating scheme that either does not require removal, or that can be removed without causing contamination or damage to the semiconductor structure. What is also needed is an antireflective coating scheme that facilitates a better photoresist profile and better control of critical dimensions due to better prevention of reflected light than is found in the prior art. What is also needed is an antireflective coating scheme that, while resisting reflecting light, resists foot poisoning of the photoresist during processing.
Antireflective structures according to the present invention comprise a metal silicon nitride composition in a layer that is superposed upon a layer to be patterned that would other wise cause destructive reflectivity during photoresist patterning. The antireflective structure has the ability to absorb light used during photoresist patterning. The antireflective structure also has the ability to scatter unabsorbed light into patterns and intensities that are substantially ineffective to photoresist material exposed to the patterns and intensities.
Preferred antireflective structures of the present invention comprise a semiconductor substrate having thereon at least one layer of a silicon-containing metallic or silicon-containing metal nitride. The antireflective layer either absorbs reflected light or dissipates reflected light into patterns and intensities that do not substantially alter photoresist material that is exposed to the patterns and intensities. The semiconductor substrate will preferably have thereon a feature size with width dimension less than about 0.5 microns, and more preferably less than about 0.25 microns.
An antireflective structure according to the present invention comprises an antireflective layer that resists fouling of the semiconductor structure such as photoresist foot poisoning and that has the ability to absorb light or to scatter light into patterns and intensities that do not substantially effect photoresist material that is exposed by those patterns and intensities.
One preferred material for the inventive antireflective layer includes metal silicon nitrides. The metal silicon nitrides are of the general formula MxSiyNz wherein M is at least one transition metal, x is less than y, and z is in a range from about 0 to about 5y. Preferably, the Si will exceed M by about a factor of two. Addition of N is controlled by the ratio in the sputtering gas used in physical vapor deposition (PVD) to deposit the metal silicon nitride material, such as Ar/N.
Minimum reflectivity may be manipulated by adjusting the thickness of the antireflective layer. Minimum reflectivity may also be manipulated by nitrogen content in the inventive antireflective layer.
Tungsten is a preferred transition metal in the fabrication of the inventive antireflective coating. A preferred tungsten silicide target for the PVD process will have a composition of silicon between 1 and 4 in stoichiometric ratio to tungsten.
The inventive antireflective layer is amorphous or has a preferable grain size that is less than the film thickness of the antireflective layer. A grain size that is substantially the same or larger than the film thickness of the inventive antireflective layer will cause a substantially discontinuous film to form. A substantially discontinuous film will detrimentally allow for reflected light to escape from the metallization layer that is to be patterned.
Composite antireflective layers made of metal suicides or metal silicon nitrides may be fashioned according to the present invention depending upon a specific application.
Another type of composite antireflective layer may be made according to present invention in which antireflective layers made of metal silicides or metal silicon nitrides may be combined with rough or hemispherical grained polysilicon. In this embodiment, it may be advantageous to use the polysilicon as a later-used conductive layer such as the conductive material in a word line or as an etch stop structure.
The reflectivity exhibited by antireflective structures of the present invention can be described as the fraction of incident light energy that escapes from the surface of the antireflective structure when irradiated by photoresist patterning light under normal operating conditions.
In connection with preferred materials and preferred reflectivities of selected structures, it is also useful to describe the present invention in terms of a variance from the design geometry of an actual characteristic geometry of the structure being fabricated. It can be appreciated that, as integrated circuit device geometries continue to shrink, the variance preferably either remains relatively constant or must also shrink.
The method of the present invention may be used to form various structures such as metallization layers. It is to be understood that the discussion of metallization layers is merely illustrative and not limiting of the inventive method. For example, isolation trenches, contact corridors, vias, stacked storage node wells, and wiring trenches arc further non-limiting examples of sties that may also be formed by the inventive method and by use of the inventive antireflective structure.